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<br />_ storage-discharge relationships for each pond. <br />} <br />Pond A <br />• <br />For purposes of design of riprap improvements to the Pond A spillway, the original 100-year hflow <br />hydrograph developed by Leonard Rice Consulting Water Engineers (LRCWE) and shown in the as-built <br />plans was used in flood routing. This hydrograph was used in the original certification of the pond s~ ilway <br />and was developed for AMC III conditions. Subsequent modrfiwtions to the pond in terms of .rater <br />conservation storage have been previously approved based on AMC II conditions. Consequently, t:se of <br />the original 100-year hydrograph, that has a peak discharge of 763 cfs and a runoff volume of 387 acr=feet, <br />is quite conservative from the standpoint of flow over the emergency spillway. <br />The 10-year flood hydrograph was developed by LRCWE to support a Technical Revision cf the <br />Sedimentation Pond A Design. This technical revision proposal was approved in January 1983. The-efore, <br />the inflows to Pond A have not been revised for this analysis and represent approved hydrology. The <br />following are the runoff volumes and peak inflows to Pond A as developed from previous analyses cf the <br />Pond A watershed, <br />Event 8 Duration Runoff Volume (AF) Peak Inflow (cfs) <br />10-year, 24-hour 52.31 78 <br />100-year, 24-hour 387 797 <br />1 As provided in LRCWE letter of 23 November 1981. <br />• <br />Table 1 shows the discharges computed for the principal outlet works using the aforemeroned <br />spreadsheet model. Spillway discharges were determined using the geometric information for the sp~'•Nays <br />shown in the as-built plans. Table 2 shows the combined stage -discharge relationship for P:-.d A. <br />Colorado Yampa Coal Company provided a spoil spring discharge of 3.8 cfs based or. t999 <br />monitoring information. To account for this flow in the hydrologic routing, the initial water surface elevation <br />in Pond A was established at the elevation corresponding to this flowrate (6936.1 feet). This assumes that <br />steady state conditions have been reached with respect to the spoil spring discharge. Steady state <br />conditions (inflow ~ outflow) for an inflow of 3.8 cfs are reached when the pond water surface ele+r:ion <br />2 <br />Water Engineering & Technology, Inc. <br />